MSK - Development Of The Limbs Flashcards Preview

What do somites develop into? (3)

Dermatone - skinMyotome - musclesSclerotome - bones

2

What is a somite?

Organisation of paraxial mesoderm into segments.Somites are bilaterally paired blocks of paraxial mesoderm that form along the head to tail axis of the developing embryo. 1st appear at day 20 in the occipital region.31 in total

3

When do limb buds become visible?

End of fourth week of development.

4

Where do they become visible?

Ventro-lateral body wall

5

Which limb bud appears first? Upper or lower limb bud?

Upper limb bud appears first followed by the lower limb bud 1 to 2 days later.

6

What do the limb buds consist of?

Mescenchymal core covered by layer of cuboidal ectoderm

7

Where is the anterior and posterior axis in the embryo?

Anterior towards the headPosterior towards the tail

Remember in the adult Superior = headAnterior = Tail

8

What is the proxio-distal axis?

Base of the limb to tips of the digits

9

Where does the limb bud come from?

Activation of mesenchyme within lateral mesoderm.Derived from the somatic later of lateral plate mesoderm

10

What is the difference between Somatic and Splanchic

Somatic - to do with the bodySplanchic - to do with viscera

11

What contributions to the limb skeleton?

Lateral plate mesoderm

12

How does the limb bud elongate?

Proliferation of the mescenchyme core. Lots of tissue created not well differentiated

13

What is the name of the thickened ectoderm at the apex of the limb bud?

Apical ectodermal ridge (AER)

14

What is the importance of the apical ectodermal ridge?

Critical for limb bud outgrowthOrchestrates limb development proximal to distalFinal stage is appearance of paddles AER then regresses - gradually at first then almost entirely.

15

What does the apical ectodermal ridge do?

Exert an inductive influence on the adjacent mesenchyme, causing it to remain undifferentiated

16

As the limb grows, what happens to the cells furthest from the apical ectodermal ridge (AER)?

Proximal mesenchyme begins to differentiate into cartilage and muscle.

Molecular signals from the AER can only communicate to cells that are closest to it so the furthest part differentiates.

17

What would happen in the absence of the apical ectodermal ridge (AER)?

The limb bud would differentiate

18

What induces the development of the digits within the hand/foot plates?

Apical Ectodermal Ridge (AER)

19

How are hand and foot plates formed?

Limb buds become flattened

20

How is the boundary between the dorsal and ventral limb ectoderm marked?

Apical ectodermal ridge

21

What regulates the patterning of the anteroposterior axis of the limb?

Zone of polarising activity

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What is the zone of polarising activity (ZPA) and role?

A cluster of mesenchyme all cells at the posterior border of the limb near the AER. The cells have an important signalling role at the posterior base of the limb bud. Controls patterning. It ensures the AER is maintained until it is no longer neededGenerates asymmetry in the limbs

23

What happens to mesenchyme to form the 'digital rays' hand and foot plates?

Mesenchyme condensations within plates.Cartilaginous models of the digital bones

24

How are fingers and toes formed?

Apoptosis (cell death) in the AER seperates the ridge into five parts. Interdigital spaces progressively sculpted by programmed cell deathFurther formation of the digits depends on their continued out growth under the influence of the five segments of ridge ectoderm, condensation of the mesenchyme to form cartilaginous digital Ray's and the death of intervening tissue between the rays

25

Which limb, upper or lower, is effected by limb defects more often?

Upper limb

26

What causes limb defects?

Hereditary but teratogens induced defects have been described

27

Name four common limb defects.

What happens in each defect?

Syndactyly - fusion of the digits, don't get sculpting of digital rays. Degree of fusion varies.Polydactyly - extra digits on either hand or foot, genetic recessive traitAmelia - complete absence of a limbMeromelia - partial absence of a limbPhocomelia - exposure to thalidomide, lost extension of limb bud

When does endochondral ossification begin?

During the end of the embryonic period

What happens during ossification?What mesoderm is involved?

What type of ossification occurs in long bones?

Endochondral ossification

31

Describe primary ossification. When are the centres present?

Primary ossification centres are present by the 12th week of development. From the primary centre in the shaft/diaphysis of the bone, endochondral ossification gradually progresses towards the ends of the cartilaginous model.

32

Where do secondary ossification centres appear?

Epiphyses

At birth, diaphysis is usually completely ossified but the two ends (epiphyses) are still cartilaginous. Shortly after ossification centres arise.

33

Why are the epiphyseal growth plates important?

Epiphyseal growth plates are important in the growth in length of bones. Allows bone to grow after birth

34

How do muscles begin to form?

Myogenic precursors migrate into limb buds from somites, they go either side of the developing limb bud

35

What are the two common muscle masses around the newly formed skeletal elements?

Ventral = flexorDorsal = extensor

36

How are individual muscles formed?

Explain segmental innervation of nerves.

They split from the common muscle masses. They bring the nerve corresponding to that body segment which explains segmental innervation of nerves.